This invention relates to a double-pass, product-staged reverse osmosis separation system, particularly suited for a water demineralization system.
Reverse osmosis membrane systems have been known for years as a means for concentrating and separating dissolved solids (salts, organic matter) from liquid solutions. The reverse osmosis membrane serves to concentrate the materials in the "concentrate flow," i.e., the stream which does not pass through the membrane, while the separation occurs in the "product flow," i.e., the stream which passes through the membrane If concentration of the materials is desired, persons in the trade tend to call the process a concentration process. If dissolved material separation from the liquid in the product stream is desired, persons in the trade tend to call the process a purification process. By either name, this process utilizes both a concentrate flow and product flow.
When the terminology "reverse osmosis" is employed herein, it is in the known sense of applying pressure greater than the natural osmotic pressure created by the dissolved matter in the solution, and opposite thereto, across a semipermeable membrane. In the absence of this applied pressure, the natural osmotic pressure would cause the purer liquid to flow through the membrane toward the more concentrated liquid in a natural tendency for the system to try to reach equilibrium, i.e., to reach the same concentration of materials on both sides of the membrane. This natural osmotic pressure varies in amount depending upon the dissolved materials in the liquid, and the concentration thereof. The greater the concentration, the greater the osmotic pressure. The terminology "product stream" or "product flow" as used herein means that stream which has passed through the semipermeable membrane, whether the purpose of the process is to purify the product stream or to concentrate the concentrate stream. The term "concentrate stream" or "concentrate flow" is utilized herein to refer to that portion of the liquid stream which does not pass through the membrane. The term "cross flow reverse osmosis" is intended in its normal sense acceptable in the trade, i.e., where the flow of the inlet feed water is generally parallel to the membrane, such that the product flow has changed direction to pass through the membrane an through a product stream outlet therefrom, and the concentrate flow has continued to pass generally parallel to the membrane and out through a concentrate stream outlet therefrom.
The term "overproduction" when used herein is intended to mean the result of an excessively high net driving pressure causing an excessively high flux rate, the amount of water (carrier liquid) that passes through a square foot of available membrane Because industrial applications involve high salt content solutions (3% or higher), the desired removal of dissolved salts and the like from a solution often cannot be achieved by a single piece of equipment, i.e., a single pass through a membrane or an ion exchange bed. Thus, it is known that 90%-plus salt removal can be achieved by reverse osmosis units in series, or using a reverse osmosis/deionizer ion exchange resin exchanger.
Ion exchange resins are expensive, typically taking the form of polymeric beads produced from petroleum. Such resin units require frequent costly regeneration, typically using either hydrochloric acid or sulfuric acid and caustic soda. Subsequent disposal of the exhausted regenerant from the acid and caustic soda is troublesome and costly.
The other potential technique for desalting solution processes would be to utilize a two step process using a primary reverse osmosis membrane unit followed by a second reverse osmosis membrane system. However, the cost of such a dual system made according to traditional technology is exorbitant, requiring separate expensive transfer pumps, piping, controls, extra collection tanks, etc., often involving a hydraulic "nightmare," particularly when one of the units starts fouling. In controlling these systems, the feed pressure across the reverse osmosis membrane is limited to prevent "overproduction," i.e., producing more water than the available membrane surface area can properly handle. Therefore, it is normally necessary to provide a first pump, carefully regulated for the first reverse osmosis unit or set of reverse osmosis membranes, and a second pump for the second reverse osmosis unit or set of reverse osmosis membranes, which is regulated relative to the product output from the first reverse osmosis unit or first set of membranes and operates at an acceptable pressure for the membranes in the second reverse osmosis unit. The resulting arrangement is highly expensive and, as noted previously, results in a complex hydraulic setup which is difficult to design, to preset, and to control. The reason for the second pump upstream between the first and second unit is that product flow, i.e., product stream, emitted from the first unit or bank of membranes has lost its pressure due to flow through the first set of membranes. If the product flow is to be forced through the second stage membrane, it must be repressurized sufficiently to overcome the natural osmotic pressure before any liquid passes through the membrane.
In operating a reverse osmotic system, the pressure of the concentrate stream is not significantly lower than the initial feed pressure. However, the pressure of the product stream is negligible due to its having passed through the membrane.